Interpretive Summary: We use photosynthetic response curves to insect herbivory intensity (% tissue removal from a single leaf) as a more refined assay of plant physiological tolerance to a stress like injury. We studied N. oleander because it has large constitutive investment in cardiac glycoside defense compounds, so that few herbivores can breach the defenses of this plant. We used clipping injury with two experiments, and actual herbivory with southern armyworm caterpillars in a third experiment, to examine the shape of photosynthetic response curves, and examine other gas exchange and chlorophyll a fluorescence parameters to better understand how photosynthetic activity is reduced on remaining tissue of injury leaves (since not all plant species experience photosynthetic reductions on remaining tissue of injured leaves). We found that plants had a negative linear relationship between photosynthetic parameters and leaf injury intensity in one study (equal drop in photosynthesis for each unit of injury), but in two other studies plants had negative exponential relationships to indicate low photosynthetic tolerance to injury (large decreases at low injury levels, and small or no additional decreases with additional injury). Results with other chlorophyll a fluorescence and gas exchange parameters suggest that injury causes mesophyll limitations to photosynthesis that reduces light harvesting efficiency of photosynthesis and subsequent energy allocation to photosynthetic light reactions. Our results are of interest to those who study a trade-off between photosynthesis and chemical defense, because chemical defense induction after injury would be expected to cause mesophyll limitations to photosynthesis. Thus, oleander and other members of Apocynaceae (e.g., dogbanes) and Asclepiadaceae (milkweeds) may serve as model species for studying chemical defense trade-offs with maintaining photosynthesis after leaf defenses are breached.